I'll do that as soon as I figure out where side-loaded books go. I dropped one into the 'books' folder, but I can't get it to show up in the reader. (One of the questions in your link is 'Tell us where books need to go'

The ebooks go in the Documents folder
It does not seem like subfolders work, and the new ebooks took a minute or so to show up.

Actually, I've been putting them in the books folder, and half of them show up in the reader, while those that don't show up under the docs tab, despite being in the books folder. See this thread for more discussion:

In that thread that I linked, I explain my experiences with .mobis. Half seem to go to books, half to docs, despite all being in books folder. I'm beginning to suspect that all is not well with Amazon's overlay implementation.

I'd like confirmation on whether sub folders work or not, as I would be fairly surprised if they did not. Also could someone create a news download in calibre (with output profile set to Kindle) and copy the resulting mobi to the fire and see what happens.

I'd like confirmation on whether sub folders work or not, as I would be fairly surprised if they did not. Also could someone create a news download in calibre (with output profile set to Kindle) and copy the resulting mobi to the fire and see what happens.

Sub folders work, Kovid. I had [PDOC] set as a conversion option under mobi output. This caused the Fire to read it as a Document, rather than book. Reconversion with this removed causes proper behavior. I'll be back in a second with the news result.

It's "Documents", but that's not where you want books to go, you want them in "Books", so they'll go into the reader (This is significant because you're then pressing the 'books' tab to find side-loaded books, rather than the 'Documents' tab.)